What this test measures
Thyroid Stimulating Hormone (TSH) is produced by the pituitary gland and tells the thyroid how much T3 and T4 to make. Because of the feedback loop between pituitary and thyroid, TSH responds before the actual thyroid hormones move — so a small change in thyroid function shows up as a large, easily detected change in TSH.
When the thyroid is underactive (hypothyroidism), the pituitary senses too little hormone and pushes TSH up. When the thyroid is overactive (hyperthyroidism), the pituitary backs off and TSH drops. That is why "third-generation" TSH (which can detect levels as low as 0.01 mIU/L) is the gold-standard first-line screen for any suspected thyroid problem.
Why it matters
Thyroid disorders are remarkably common in India — estimated to affect 1 in 10 adults, with women three times more likely than men. Hypothyroidism in particular is under-diagnosed: symptoms (fatigue, weight gain, hair loss, cold intolerance, low mood) are non-specific and often dismissed. Even mild ("subclinical") hypothyroidism in pregnancy can affect fetal brain development, which is why TSH is now part of every antenatal screen in India.
TSH testing is also used to monitor people already on thyroid hormone replacement (levothyroxine) — the goal is to keep TSH in the lower half of the normal range, and dose adjustments are made every 6–8 weeks based on TSH alone.
How to prepare
No fasting required. TSH can be tested at any time of day, but values are slightly higher in the morning so consistent timing makes comparison across visits easier. If you are on thyroid hormone (levothyroxine), take it after the blood draw on test day — taking it before can transiently inflate the T4 reading (TSH itself is not affected as quickly). Continue all other medications unless your doctor tells you otherwise.
Note: biotin supplements (often in skin / hair / nail products at high doses) can interfere with many TSH assays and produce falsely low TSH. Stop biotin for 48–72 hours before the test if you take it.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| TSH (3rd Generation) (mIU/L)[1][2][3] | 0.4 – 4.0 (general adult) · 0.1 – 2.5 (1st trimester pregnancy) · 0.2 – 3.0 (2nd & 3rd trimester) | Low TSH = the pituitary is signalling that there is enough or too much thyroid hormone. Causes: hyperthyroidism (Graves' disease, toxic multinodular goitre, thyroiditis), over-replacement with levothyroxine, recent iodinated contrast, biotin interference. Suppressed TSH (<0.01) usually means hyperthyroidism; check Free T4 and Free T3 to confirm. | High TSH = the pituitary is pushing harder because the thyroid is not making enough hormone. Causes: primary hypothyroidism (most often autoimmune / Hashimoto's in India), inadequate levothyroxine replacement, recovery phase of thyroiditis, iodine deficiency in some regions, certain drugs (lithium, amiodarone). Mild rises (5–10 mIU/L) are common and often called "subclinical hypothyroidism". |
How to read your TSH result
| TSH (mIU/L) | Status | What it usually means | Next steps |
|---|---|---|---|
| < 0.1 | Suppressed | Overt hyperthyroidism or over-replacement | Check Free T4 and Free T3; clinical evaluation |
| 0.1 – 0.39 | Low (mild) | Subclinical hyperthyroidism or early hyperthyroidism | Repeat in 6–8 weeks; check Free T4; consider antibodies |
| 0.4 – 4.0 | Normal | Thyroid likely functioning normally | Annual re-check if symptomatic or on therapy |
| 4.1 – 9.9 | High (mild) | Subclinical hypothyroidism | Repeat in 6–8 weeks; check anti-TPO; treat if pregnant, planning pregnancy, or symptomatic |
| ≥ 10.0 | Significantly high | Overt primary hypothyroidism | Start levothyroxine; re-check in 6–8 weeks; check anti-TPO |
Frequently asked questions
Do I need to fast for a TSH test?
No. TSH does not require fasting and can be done at any time of day. For consistency across visits, many doctors recommend testing at the same time of day each time (morning is most common).
I am on thyroid medication — when should I take it on the test day?
Take your levothyroxine after the blood draw, not before. Taking it 1–2 hours before the test can transiently raise the T4 reading (though TSH itself is more stable). If you forgot and took it, mention it to your doctor so the result is interpreted correctly.
My TSH is 5.5 — am I hypothyroid?
A TSH between 4.0 and 9.9 with normal Free T4 is called "subclinical hypothyroidism" — your thyroid is starting to slow down but is not yet failing. Most doctors will repeat the test in 6–8 weeks. Treatment is started if you are pregnant, planning pregnancy, have symptoms, or have positive anti-TPO antibodies (autoimmune thyroiditis).
Can stress raise my TSH?
Acute illness can transiently lower TSH (sick euthyroid syndrome). Chronic stress does not directly raise TSH. If your TSH is unexpectedly off and you have been seriously ill in the past few weeks, repeat the test when you are back to baseline.
Why is the pregnancy TSH range different?
Pregnancy hormones (hCG) cross-react with the TSH receptor and lower TSH naturally, especially in the first trimester. Tighter targets (TSH < 2.5 in 1st trimester, < 3.0 in 2nd / 3rd) protect fetal brain development. Pre-pregnancy and antenatal TSH testing is recommended for every Indian woman.
How is TSH different from a Thyroid Profile (T3, T4, TSH)?
TSH alone is the screening test — sensitive enough to pick up most thyroid problems. A full Thyroid Profile (T3, T4, TSH or Free T3, Free T4, TSH) is ordered when TSH is abnormal, when symptoms strongly suggest thyroid disease but TSH is borderline, or to monitor specific conditions like Graves' disease and pituitary disorders.
Should I take biotin / hair-skin-nail supplements before the test?
Stop biotin (B7) for 48–72 hours before the test. High-dose biotin (often 5–10 mg in cosmetic supplements) interferes with many TSH and thyroid hormone assays and can produce falsely low TSH and falsely high T4 / T3.
How often should I get a TSH test?
Once every 5 years for adults 35+ if no risk factors, annually for women planning pregnancy or with a family history, and every 6–8 weeks while a thyroid hormone dose is being adjusted. Once stable on a fixed dose, every 6–12 months is enough.
Related Hormones / Endocrine tests
Tests commonly ordered alongside THYROID STIMULATING HORMONE (TSH), or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Hypothyroidism Guidelines · accessed 2026-05-29T00:00:00.000Z
- NIH MedlinePlus — TSH Test · accessed 2026-05-29T00:00:00.000Z
- Endocrine Society Clinical Practice Guidelines — Thyroid · accessed 2026-05-29T00:00:00.000Z
- Indian Thyroid Society — Consensus on Hypothyroidism · accessed 2026-05-29T00:00:00.000Z
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